Method for removing nitrogen from an aqueous solution

ABSTRACT

A method of removing nitrogen from an aqueous solution by simultaneous microbial nitrification and dentrification comprising treating the aqueous solution in one or more fixed bed reactors containing a porous carrier material having nitrifying and denitrifying microorganisms fixed thereto, and using an ammonium selective ion exchanger as carrier material.

The present invention relates to a method for removing nitrogen from anaqueous solution, such as waste water and drinking water, bysimultaneous microbial nitrification and denitrification comprisingtreating the aqueous solution in one or more fixed bed reactorscontaining a porous carrier material having nitrifying and denitrifyingmicroorganisms fixed thereto.

Nitrification is usually effected by aerobic autotrophs in a two-stepreaction. Firstly, ammonium is converted to nitrite by e.g.Nitrosomonas, and then nitrite is converted to nitrate by e.g.Nitrobacter. The overall reaction may be represented as follows:

    NH.sub.3 +CO.sub.2 +O.sub.2 +bacteria→NO.sub.3 +new bacteria

Denitrication is effected by facultative heterotrophs under anoxiccondition and using organic carbon as electron donor. As a result of thedenitrification, nitrate ions are reduced to free nitrogen, which isliberated in gaseous form. The denitrification may be represented asfollows:

    NO.sub.3 +organic matter+bacteria→N.sub.2 (gas)+new bacteria

As will appear from the above, nitrification and denitrification takeplace under different physical conditions. However, when a porouscarrier material is used, nitrification and denitrification, i.e. theconversion of ammonium to gaseous nitrogen, can be carried outsimultaneously in one reactor. Thus, by using a porous carrier material,it is possible to establish anoxic conditions in the interior of thepores and aerobic conditions at the surface of the carrier material.

DE patent publication No. 38 10 564 Al discloses a carrier material forfixing microorganisms, particularly nitrifying and denitrifyingmicroorganisms, said carrier material being made up by a mixture ofpumice and clay having a pore size of from 0.05 to 0.5 mm.

DE patent publication No. 36 39 153 Al discloses a carrier material foruse in different types of bioreactors, which carrier material consistsof porous, inorganic beads of sinter.

The prior art carrier material may be used for removing nitrogen fromwaste water.

DK patent application No. 160/90 discloses a method of nitrification andremoval of nitrogen using nitrifying bacteria, which method comprisesimmobilizing the bacteria on a porous carrier material, such asactivated carbon, zeolite, ceramics and ion exchangers.

JP patent application no. 85/58228 discloses pellets of calcium alginategel containing clinoptilolite and immobilized nitrateforming bacteriaand nitrifying bacteria. The pellets may be used in connection with lakewater purification.

In the process of simultaneous nitrification and denitrification, theoverall nitrogen removal is usually limited by the nitrificationreaction, which is believed to be due to the fact that the specificreaction rate of the nitrification reaction is much lower than that ofthe denitrification reaction.

Furthermore, the growth rate of nitrifying bacteria is lower than thatof denitrifying bacteria, and accordingly in a heterogenous culturecontaining both nitrifying and denitrifying bacteria, there is a riskthat the denitrifying bacteria will propagate at the expense of thenitrifying bacteria, thus resulting in a reduction of the total amountof nitrifying bacteria and hence the total nitrification rate.

The object of the present invention is to provide a more efficient and amore reliable method for removing nitrogen from an aqueous solution.

The method of the invention is characterized in that an ammoniumselective ion exchanger is used as carrier material.

The invention is based on the discovery that by using an armnoniumselective ion exchanger as carrier material an extremely high nitrogenremoval rate can be achieved.

It is believed that the high removal rate achieved by the method of theinvention is the result of an increased transport of ammonium across thebiofilm as explained in the following.

The nitrifying and denitrifying bacteria are contained in a biofilmcovering the carrier material, the nitrifying bacteria being positionedin the outer layer of said biofilm and the denitrifying bacteria beingpositioned in the inner layer thereof.

The transport of ammonium across the biofilm is dependent on themagnitude of the ammonium concentration gradient across the biofilm.

When an ammonium selective ion exchanger is used as carrier material,part of the ammonium molecules crossing the biofilm will be absorbedonto the ion exchanger, and part of the absorbed ammonium molecules willcontinuously be desorbed from the ion exchanger and taken up by thenitrifying bacteria.

The ammonium molecules absorbed on the ion exchanger, which moleculesare being replaced continuously by new ammonium molecules, do not haveany influence on the concentration gradient across the biofilm, i.e. thetransport driving force, the result being an increased overall ammoniumtransport across the biofilm.

A further advantage of the method of the invention is that when themethod is being used for removing nitrogen from aqueous solutionscontaining both ammonium and nitrate, which is the case with many typesof waste water, the above discussed increased ammonium transport acrossthe biofilm will result in an increased transport of nitrate as well,the oppositely charged ionic molecules acting as counter ions during thetransport.

The ammonium selective ion exchanger used in the method of the inventionis preferably macroporous. By using a macroporous carrier a highnitrogen removal per unit of reactor volume and per unit of time can beachieved. This is i.a. due to the fact 1) that a macroporous carriermaterial provide a large surface area for the fixing of microorganismsand thus a high concentration of microorganisms, and 2) that thenitrification and denitrification take place within micrometers fromeach other, thus reducing the time of transport of the nitrate betweenthe site of nitrification and the site of denitrification to a minimum.

Examples of suitable ammonium selective ion exchanger areclinoptilolite, phillipsite, mordenite and irionite.

Preferably, activated clinoptilolite is used as carrier material.Clinoptilolite is activated by replacing the potassium ions of thenatural clinoptilolite with sodium ions, the latter having a highpotential for being exchanged for ammonium ions.

A preferred embodiment of the method of the invention is characterizedin that the ammonium selective ion exchanger is inoculated withnitrifying microorganisms, which are allowed to propagate before thesupply of the aqueous solution to the reactor is initiated.

The ion exchanger is inoculated with nitrifying microorganisms in orderto ensure a high starting concentration of said microorganisms and hencea high concentration of the microorganisms during the subsequentpurification process.

When the ammonium selective ion exchanger is inoculated with nitrifyingmicroorganisms, the propagation of the inoculum of nitrifyingmicroorganisms is preferably carried out in a buffer solution, e.g. aphosphate buffer solution, in order to prevent a lowering of theph-value of the solution surrounding the microorganisms, such a loweringhaving an adverse effect on the growth of the microorganisms.

In another preferred embodiment of the invention, the ratio of carbon tonitrogen in the aqueous solution supplied to the fixed bed reactor iscontrolled in order to control the relative growth of nitrifying anddenitrifying microorganisms so as to secure that a high concentration ofnitrifying microorganisms is maintained at all times. The ratio ofcarbon to nitrogen (C:N) in the aqueous solution supplied to the fixedbed reactor is preferably from about 1.0 to about 6.0, more preferablyfrom about 3.0 to about 5.0.

A further preferred embodiment of the invention is characterized in thatsubsequent to the propagation of the inoculum of nitrifyingmicroorganisms, the ion exchanger is inoculated with denitrifyingmicroorganisms, e.g. in the form of activated sludge containingdenitrifying microorganisms, which are allowed to propagate before thesupply of the aqueous solution is initiated.

Still another embodiment of the method of the invention, wherebycarbonaceous matter is removed from the aqueous solution, ischaracterized in that prior to the treatment in the fixed bed reactorusing an ammonium selective ion exchanger as carrier material, theaqueous solution is treated in a separate fixed bed reactor containing acarrier material having microorganisms capable of decomposingcarbonaceous matter fixed thereto.

When carbonaceous matter is to be removed from the aqueous solution, abark ion exchanger is preferably used as carrier material in theseparate fixed bed reactor. Such a bark ion exchanger is particularlysuitable for holding bacteria capable of decomposing carbonaceous matterand hence for obtaining removal of carbonaceous matter.

When carbonaceous matter is to be removed from the aqueous solution, themethod of the invention is preferably carried out so that the carriermaterial in the separate fixed bed reactor is inoculated withmicroorganisms capable of decomposing carbonaceous matter, e.g. in theform of activated sludge containing said microorganisms, whichmicroorganisms are allowed to propagate before the supply of the aqueoussolution is initiated.

Preferably, the fixed bed reactor used in the method of the invention isan upflow column.

Alternatively, the fixed bed reactor used in the method of the inventionmay be a reactor having a progressively diminishing cross-sectional areain the flow direction of the aqueous solution.

In a reactor of this type, the flow rate of the aqueous solution willincrease through the column, and hence the hydraulic retention time willdecrease through the reactor.

Such a reactor can e.g. have the form of a box-shaped tank divided intotwo compartments by an inclined wall so as to form two compartmentshaving different volumes, the larger compartment having a larger bottomarea than the smaller compartment and the two compartments having topareas of substantially equal sizes.

The above described reactor operates as follows:

The aqueous solution is supplied to the bottom of the larger compartmentunder pressure and pumped up through said compartment to the topthereof, from where it is carried to the top of the adjacent smallercompartment and allowed to pass down through the smaller compartment,from the bottom of which the purified aqueous solution is collected.

In a traditional upflow column, the flow rate of aqueous solution isconstant through the column, and thus the amount of nitrogen supplied toa given point in the column per unit of time, which amount in thefollowing will be referred to as the nitrogen loading, will decreaserapidly through the column as more and more nitrogen is being removedfrom the aqueous solution supplied to the bottom of the column. Thus, ina traditional upflow column, the nitrogen removal rate, i.e. thenitrogen removal per unit of reactor volume and per unit of time, willdecrease up through the column.

By using a reactor having a progressively diminishing cross-sectionalarea, the decrease of the nitrogen loading in the flow direction of theaqueous solution can be reduced and thus, the efficiency of the reactorcan be improved.

The ammonium ion selectivity of a ion exchanger may be determined usingthe following test:

A suitable amount of ion exchanger is contacted with an excess amount ofan aqueous solution containing ammonium ions in a concentration of 0.1Mand sodium ions in a concentration of 0.1M. The mixture of ion exchangerand solution is stirred, e.g. by shaking.

Then, the solution is separated from the ion exchanger, and the amountof ammonium ions and sodium ions, which have been absorbed to the ionexchanger are determined, e.g. by determining the amount of ammoniumions and sodium ion removed from the separated solution.

In connection with the present invention, "ammonium ion selectivityrelative to sodium ions" is defined as the ratio of the amount ofammonium ions absorbed to the ion exchanger to the amount of sodium ionsabsorbed to the ion exchanger (NH₄ ⁺ /Na⁺) when applying the abovedescribed test.

In connection with the present invention, the term "ammonium selectiveion exchanger" is to be understood as meaning any ion exchanger havingan ammonium ion selectivity relative to sodium ions of above 1.0,preferably above 1.5, more preferably above 2.0, still more preferablyabove 2.5 and most preferably above 5.0.

In the following, the present invention will be described in furtherdetail with reference to the example below.

EXAMPLE

An aqueous solution containing from 30 to 1000 mg nitrogen per liter, inthe form of ammonium and from 0.1 to 0.70 mg nitrogen per liter in theform of nitrate was purified using the method of the invention.

The nitrogeneous aqueous solution was prepared to simulate differenttypes of waste water. The ammonium of the aqueous solution was added inthe form of ammonium chloride. The organic carbon source required in thedenitrification was added in the form of stoichiometric amounts ofethanol.

Activated clinoptilolite having a grain size of from 2.0 to 4.0 mm andmontmorillonite (40% ammonium selective relative to activatedclinoptilolite) were used as carrier materials, and for comparisonpurposes also Leca® (0% ammonium selective) was used.

The carrier material was packed in a tubular container of plexiglass andhaving an inner diameter of 200 mm and a height of 1.10 m. The fixed bedreactor had a bed volume of about 28 liters and a void volume of about 8liters. The reactor was aerated in upflow direction so as to obtain aconcentration of dissolved oxygen of between 2 and 3 mg/l (aerobicconditions).

The temperature of the reactor was 20° C. at all times, and the pH-valueof the reactor was maintained at a level of from 7.7 to 7.8.

The inoculation of the reactor with nitrifying bacteria was carried outas follows. The nitrifying bacteria was propagated in a concentratednutrient solution (TGY-medium) to obtain a cell concentration of about10⁹ cells per liter. The propagated cells were then added to a solutioncontaining 30 mg nitrogen in the form of ammonium per liter and 230 mgethanol per liter.

The resulting solution was then supplied to the fixed bed reactor, whichcontained a phosphate buffer solution having a ph-value of 6.8. Thebacteria was then allowed to propagate for a period of 2 to 3 days,during which the mixture of the supplied solution and the buffersolution was circulated and during which ammonium was continuously beingadded to the reactor to maintain a suitable concentration of ammonium atall times.

Subsequent to the propagation of nitrifying bacteria, the reactor wasinoculated with sludge containing denitrifying bacteria, which was thenallowed to propagate before the supply of the nitrogeneous aqueoussolution to be purified was initiated.

In Tables 1, 2 and 3 the results obtained are listed for activatedclinoptilolite, montmorillonite and Lecal®, respectively.

FIG. 1 shows the results listed in Tables 1, 2 and 3 (curve A, B and C,respectively) in an (X=Nitrogen loading; Y=Nitrogen removalrate)-diagram.

FIG. 2 shows the nitrogen removal rate at a nitrogen loading of 3.960 kgN/m³ ·day (Y) plotted against the degree of ammonium selectively of thecarrier material (X).

As will appear from Tables 1, 2 and 3 and from FIG. 1, the performanceof the two methods using ammonium selective ion exchangers as carriermaterial are highly superior to the method using a carrier materialhaving no ammonium selectivity.

Furthermore, it will appear that the method using activatedclinoptilolite is highly superior to the method using montmorillonite.

In the method using activated clinoptilolite, the efficiency obtained,i.e. the ratio of nitrogen removed to nitrogen supplied, is very higheven at very high nitrogen loadings. As will appear from FIG. 1, theefficiency do not show any sign of decrease within the nitrogen loadinginterval tested, i.e. up to 14,118 kg nitrogen per m³ reactor volume perday.

                                      TABLE 1                                     __________________________________________________________________________    Carrier Material: Clinoptilolite                                                                                  Nitrogen                                  Nitrogen  Nitrogen in     Nitrogen in                                                                             removal                                   loading   the form of ammonium                                                                          the form of nitrate                                                                     rate                                      Flow                                                                              kg N/m.sup.3 ·                                                             Influent                                                                           Effluent                                                                           Efficiency                                                                          Influent                                                                           Effluent                                                                           kg N/m.sup.3 ·                                                             Efficiency                          l/h day   mg/l mg/l %     mg/l mg/l day   %                                   __________________________________________________________________________    1.2 0.108  30.0                                                                              1.49 95.0  0.10 1.30 0.098 90.7                                2.0 0.178  30.0                                                                              0.22 99.3  0.10 2.12 0.165 92.7                                3.6 0.324  30.0                                                                              0.51 98.3  0.20 0.22 0.319 98.5                                0.9 0.270 100.0                                                                              1.44 98.6  0.40 4.63 0.255 94.4                                1.5 0.450 100.0                                                                              1.61 98.4  0.10 3.95 0.425 94.4                                2.0 0.585 100.0                                                                              0.70 99.3  0.70 29.70                                                                              0.412 70.4                                2.5 0.738 100.0                                                                              1.38 98.6  0.10 6.25 0.683 92.5                                5.3 1.600 100.0                                                                              1.96 98.0  0.40 1.65 1.550 96.9                                1.5 2.250 500.0                                                                              0.67 99.9  0.46 0.49 2.248 99.9                                2.6 3.960 500.0                                                                              0.71 99.9  0.42 0.30 3.956 99.9                                4.7 7.059 500.0                                                                              0.72 99.9  0.70 0.23 7.055 99.9                                0.8 2.392 1000.0                                                                             2.23 99.8  0.12 1.97 2.383 99.6                                2.4 7.100 1000.0                                                                             5.51 99.4  0.26 0.66 7.059 99.4                                4.7 14.118                                                                              1000.0                                                                             42.3 95.8  0.35 0.70 13.516                                                                              95.7                                __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Carrier Material: Montmorillonite                                                                                 Nitrogen                                  Nitrogen  Nitrogen in     Nitrogen in                                                                             removal                                   loading   the form of ammonium                                                                          the form of nitrate                                                                     rate                                      Flow                                                                              kg N/m.sup.3 ·                                                             Influent                                                                           Effluent                                                                           Efficiency                                                                          Influent                                                                           Effluent                                                                           kg N/m.sup.3 ·                                                             Efficiency                          l/h day   mg/l mg/l %     mg/l mg/l day   %                                   __________________________________________________________________________    1.2 0.108  30.0                                                                               12.5                                                                              58.3  0.10 2.50 0.054 50.0                                2.0 0.178  30.0                                                                               13.1                                                                              41.7  0.10 4.10 0.077 43.4                                3.6 0.324  30.0                                                                               15.0                                                                              34.6  0.20 3.00 0.132 40.7                                5.3 1.600 100.0                                                                               51.9                                                                              49.1  0.40 4.70 0.657 41.1                                2.6 3.960 500.0                                                                              251.0                                                                              50.2  0.42 4.80 1.937 48.9                                4.7 7.059 500.0                                                                              299.1                                                                              29.8  0.70 1.23 2.829 40.1                                __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Carrier Material: Leca ®                                                                                      Nitrogen                                  Nitrogen  Nitrogen in     Nitrogen in                                                                             removal                                   loading   the form of ammonium                                                                          the form of nitrate                                                                     rate                                      Flow                                                                              kg N/m.sup.3 ·                                                             Influent                                                                           Effluent                                                                           Efficiency                                                                          Influent                                                                           Effluent                                                                           kg N/m.sup.3 ·                                                             Efficiency                          l/h day   mg/l mg/l %     mg/l mg/l day   %                                   __________________________________________________________________________    1.2 0.108  30.0                                                                               27.0                                                                              10.0  0.20 1.20 0.007 6.6                                 2.0 0.178  30.0                                                                               27.2                                                                              9.4   0.10 0.40 0.015 8.3                                 3.6 0.324  30.0                                                                               27.8                                                                              7.3   0.15 0.50 0.020 6.2                                 5.3 1.600 100.0                                                                               91.9                                                                              8.1   0.30 0.40 0.128 8.0                                 2.6 3.960 500.0                                                                              455.0                                                                              9.0   0.40 0.60 0.354 9.0                                 4.7 7.059 500.0                                                                              460.0                                                                              8.0   0.10 0.70 0.556 7.9                                 __________________________________________________________________________

As will appear from FIG. 2, the nitrogen removal rate obtained appear tobe linearly dependent on the degree of ammonium selectively of thecarrier material.

We claim:
 1. A method of removing nitrogen from an aqueous solution bysimultaneous microbial nitrification and denitrification comprisingtreating the aqueous solution in one or more fixed bed reactorscontaining a porous material having nitrifying and denitrifyingmicroorganisms fixed thereto, said carrier material consisting of anammonium selective ion exchanger.
 2. A method according to claim 1,wherein said ion exchanger has an ammonium ion selectively relative tosodium ions of above 1.0.
 3. A method according to claim 1, wherein saidion exchanger is a microporous ammonium selective ion exchanger.
 4. Amethod according to claim 1, wherein said carrier material is selectedfrom the group consisting of clinoptilolite, phillipsite, mordenite andorionite.
 5. A method according to claim 4, wherein said carriermaterial is activated clinoptilolite.
 6. A method according to claim 1,wherein said ammonium selective ion exchanger is inoculated withnitrifying microorganisms, which are allowed to propagate before thesupply of the aqueous solution to the reactor is initiated.
 7. A methodaccording to claim 6, wherein that the propagation of the inoculum ofnitrifying microorganisms is carried out in a buffer solution.
 8. Amethod according to claim 1, including the step of controlling the ratioof carbon to nitrogen in the aqueous solution supplied to the fixed bedreactor.
 9. A method according to claim 8, wherein that the ratio ofcarbon to nitrogen is from about 1.0 to about 6.0.
 10. A methodaccording to claim 8, wherein that the ratio of carbon to nitrogen isfrom about 3.0 to about 5.0.
 11. A method according to claim 1, whereinsubsequent to the propagation of the inoculum of nitrifyingmicroorganisms, the ion exchanger is inoculated with denitrifyingmicroorganisms, which are allowed to propagate before the supply of theaqueous solution is initiated.
 12. A method according to claim 1,whereby carbonaceous matter is removed from the aqueous solution, andwherein prior to the treatment in the fixed bed reactor using anammonium selective ion exchanger as carrier material, the aqueoussolution is treated in a separate fixed bed reactor containing a carriermaterial having microorganisms capable of decomposing carbonaceousmatter fixed thereto.
 13. A method according to claim 12, wherein saidcarrier material in the separate fixed bed reactor is a bark ionexchanger.
 14. A method according to claim 12 wherein that the carriermaterial in the separate fixed bed reactor is inoculated withmicroorganisms capable of decomposing carbonaceous matter, whichmicroorganisms are allowed to propagate before the supply of the aqueoussolution is initiated.
 15. A method according to claim 12, wherein saidfixed bed reactor is an upflow column.
 16. A method according to claim12, wherein said fixed bed reactor has a progressively diminishingcross-sectional area in the flow direction of the aqueous solution. 17.A method according to claim 16, wherein said fixed bed reactor has theform of a box-shaped tank divided into two compartments by an inclinedwall.